1. Field of Invention
The present invention is related to an LED lighting device, and more particularly to a lighting device which needs to comply with the safety standards of minimum creepage distances.
2. Related Art
Currently, for achieving the requirements of energy saving and environmental protection, the manufactures and the research & development unit of the lighting devices are gradually adopting LEDs as the light source of lighting devices. LED is a solid-state semiconductor device that can convert electrical energy directly into light. The LEDs have the advantages of small volume, fast response speed, low power consuming, and low pollution.
However, the lighting devices belong to electronic products. Safety standards are required to be considerate to ensure that personal safety wouldn't be endangered and property and environment wouldn't be harmed in use. For the safety standards of LEDs, different countries enacted different standards for different working voltages.
IEC (International Electrotechnical Commission) regulates the safety standards of the lighting products under a working voltage of 1000V in IEC 60598-1 (Luminaires—Part 1: General requirements and tests), where standards of creepage distance for different working voltage are defined. Creepage distance is the shortest path between two conductive parts (or between a conductive part and the bounding surface of the equipment) measured along the surface of the insulation. If the creepage distance is not enough, it may incur dielectric breakdown caused by electric leakages or insulation failure.
In order to solve the above mentioned problems, most of designers may drive LEDs in parallel, so that the working voltages of the whole system can be lower than Safety Extra Low Voltage (SELV). The SELV means that a voltage between conductors or between any conductor and earth does not exceed the standard voltage. No values of creepage distance are specified for working voltages below SELV. Therefore, when the working voltage of a lighting device is below SELV, the creepage distance is not needed to be considered. Moreover, different countries may have different standards for the SELV. However, because the internal resistances of each LED are different, parallel connection may cause a problem of uneven electric current, which may affect the lighting uniformity of the lighting devices and lower the life span of the LEDs. In addition, in order to solve the problem of uneven electric current for each LED, an additional current control IC is used for controlling the electric current of each LED by the manufactures. Nevertheless, as the demand for wattage is increased, the number of LED is increased; therefore, the number of ICs is increased as well as manufacturing costs, so that it is not a perfect way to solve the problem.
Therefore, how to improve the structure of lighting device so that the lighting device can meet the standard requirements for creepage distances without increasing manufacturing costs is the problem to be solved.